1) Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic system like a copying machine, a printer, a facsimile, or a multifunction product including functions of the foregoing, and a charging device and a process cartridge that are used in such an image forming apparatus.
2) Description of the Related Art
Conventionally, image forming apparatuses like a copying machine, a printer, and a facsimile adopting an image forming method according to the electrophotographic system have been widespread. Image formation according to the electrophotographic system is performed as described below. First, an outer peripheral surface of a photosensitive member, on which a resistance and the like change according to a degree of light irradiation, is uniformly charged, and a laser beam or the like is irradiated on the outer peripheral surface of the photosensitive drum, whereby an electrostatic latent image corresponding to an output image is formed thereon. Then, a charged toner is deposited on the electrostatic latent image formed on the outer peripheral surface of the photosensitive member to develop the electrostatic latent image and form a toner image. Subsequently, the toner image formed on the outer peripheral surface of the photosensitive member is transferred onto a recording medium, and heat and pressure are applied to the recording medium on which the toner image is transferred, whereby the toner image is fixed on the recording medium.
As a method of charging the outer peripheral surface of the photosensitive member, a corona charging method, a contact charging method, a non-contact charging method, and the like are known. In the corona charging method, a wire of tungsten or the like is disposed in parallel with the photosensitive member along a longitudinal direction of the photosensitive drum, corona discharge is caused by applying a high voltage of several kilovolts to the wire to ionize elements in the air around the wire, and the ions are deposited on the outer peripheral surface of the photosensitive member. In the contact charging method, a charging roller is arranged in contact with the outer peripheral surface of the photosensitive member, corona discharge is caused in a gap portion between the charging roller and the photosensitive member by applying a voltage to the charging roller, and the outer peripheral surface of the photosensitive member is charged by this corona discharge. In the non-contact method, a charging roller is arranged with a very small space between the charging roller and the outer peripheral surface of the photosensitive member, corona discharge is caused in a gap portion between the charging roller and the photosensitive member by applying a voltage to the charging roller, and the outer peripheral surface of the photosensitive member is charged by this corona discharge.
According to the corona charging method, corona discharge is caused in the wire and ions generated by this corona discharge are deposited on the outer peripheral surface of the photosensitive member. Thus, electric power used for charging the photosensitive member is only 5 to 30% of electric power supplied to the wire, and effective use of the electric power is not realized. On the other hand, according to the contact charging method and the non-contact charging method, since corona discharge occurs between the charging roller and the photosensitive member, it is possible to realize effective use of electric power.
In addition, as disclosed in Japanese Patent Application Laid-Open No. S60-95459 and Japanese Patent Application Laid-Open No. H11-305522, the inventions are proposed which supplies a low oxygen gas to an area where, corona discharge is performed to thereby reduce ozone (O3) and nitrogen oxides (NOx) which are generated when the air in the atmosphere is ionized according to corona discharge. Conventionally, in this type of image forming apparatuses using the electrophotographic system, a contact-type charging device adopting a charging roller instead of a corona charging device is often used for the purpose of limiting generation of ozone and nitrogen oxides (NOx) in a charging process.
However, even in such a charging device adopting a charting roller, ozone and nitrogen oxides are still generated in a charging processing, although quantities thereof are not so large compared with those in the corona charging device. Since a discharge area is near the surface of a photosensitive member (image bearing member), when a corona product like ozone is generated, the corona product is deposited on the surface of the photosensitive member to deteriorate the surface. As a result, blots and blurs occur in an image formed on the photosensitive member to deteriorate an image quality.
To solve these problems, Japanese Patent Application Laid-Open No. H11-305522 and the like disclose a technique for closing an outer periphery of a charging roller with a cover and supplying a low oxygen gas generated outside the charging roller to the inside of the cover to thereby reduce a corona product such as ozone in the discharge area.
The conventional image forming apparatuses described above have a problem in that the image forming apparatuses are large and expensive. In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H11-305522, it is necessary to set a large low oxygen gas generator for generating a low oxygen gas outside the cover closing the outer periphery of the charging roller. Therefore, the image forming apparatus is inevitably large and expensive.
In addition, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H11-305522, a low oxygen gas with a predetermined flow rate is injected into the cover. Thus, when air tightness in the cover is not secured, it is likely that the low oxygen gas leaks from the cover to cause a large air flow that scatters a toner into the image forming apparatus. Thus, when the flow rate is controlled to prevent the large air flow from being caused, since a predetermined low oxygen concentration cannot be obtained unless air tightness in the cover is increased, a sealing structure for closing the charging roller is complicated and assemblability of the image forming apparatus is hindered.
When the low oxygen gas is supplied, the low oxygen gas is supplied after moisture is removed as much as possible as impurities to prevent moisture condensation in a gas supply device. Consequently, the low oxygen gas is in an extremely low humidity state. Thus, when the charging device is used under a low humidity environment, abnormal discharge tends to occur, charging abnormality of the photosensitive member is caused by the abnormal discharge, and an image quality of an image to be formed is deteriorated.
Moreover, according to the conventional contact charging method and non-contact charging method, discharge is performed between the charging roller and the photosensitive member. Thus, an impact of the discharge acts on the outer peripheral surface of the photosensitive member, and the outer peripheral surface of the photosensitive member tends to be deteriorated by the impact of the discharge. In particular, when a voltage obtained by superimposing an AC voltage of a peak-to-peak voltage about twice as large as a breakdown voltage on a DC voltage is applied to make charging on the photosensitive drum uniform, the number of times of discharge increases significantly according to a frequency of the AC voltage, the impact acting on the photosensitive member increases significantly according to the discharge, and the deterioration of the photosensitive member becomes noticeable.
According to the inventions disclosed in Japanese Patent Application Laid-Open No. S60-95459 and Japanese Patent Application Laid-Open No. H11-305522, since a gas with an oxygen concentration lower than that of the atmosphere is supplied to the discharge area, it is considered that an amount of generation of ozone and nitrogen oxides can be reduced. However, since the inventions do no consider a voltage to be applied for discharge at all, deterioration of the photosensitive member due to the impact of discharge cannot be reduced. Moreover, when the oxygen concentration of the gas supplied to the discharge area is lowered, the breakdown voltage falls according to the Paschen's law when a ratio of nitrogen increases following the decline of the oxygen concentration. Thus, if the voltage to be applied for discharge is not considered at all, the impact of discharge on the photosensitive member increases and the deterioration of the photosensitive member increases.